Dutch Elm Disease 



451 



D. E. PARKER is an assistant division 

 leader of the Division of Forest Insect 

 Investigations, Bureau of Entomology 

 and Plant Quarantine. A graduate of 

 the University of Massachusetts, Mr. 

 Parker joined the Department in 1925. 

 After 9 years in Massachusetts, where 



he worked on biological control of 

 forest insects, he began studying the 

 relation of insects to tree diseases, par- 

 ticularly Dutch elm disease and elm 

 phloem necrosis. In this connection 

 Mr. Parker spent three years studying 

 the Dutch elm disease in Europe. 



DUTCH ELM DISEASE 



R. U. SWINGLE, R. R. WRITTEN, E. G. BREWER 



The Dutch elm disease is caused 

 by the fungus Ceratostomella ulmi. 

 The disease was discovered in the 

 Netherlands 30 years ago and it spread 

 rapidly in Europe. It was found in the 

 United States in 1930; it had been 

 brought here in elm burl logs imported 

 for the veneer industry. 



Native elms of the United States are 

 dangerously susceptible to the fungus. 

 Despite vigorous efforts to suppress it, 

 the disease has become established in 

 plantations and natural stands of the 

 principal elm shade-tree areas of this 

 country from Boston as far westward as 

 Indiana and Kentucky and southward 

 to Virginia. It has been found in Ten- 

 nessee. An isolated outbreak was dis- 

 covered in Colorado. 



DUTCH ELM DISEASE produces a wilt- 

 ing or yellowing of leaves on one or 

 several branches. Thereupon the leaves 

 fall. Later in the season or in follow- 

 ing years, the disease may spread to 

 other parts of the tree until the entire 

 top is affected and the tree dies. In 

 more acute cases, the entire tree may 

 suddenly wilt and die with or without 

 pronounced yellowing of foliage. In 

 all cases of Dutch elm disease, a dis- 

 coloration of the sapwood occurs in 

 affected branches, trunk, and roots. If 

 Dutch elm disease is present, a diag- 

 onal cut through branches with wilted 

 or yellowing leaves will show brown 

 spots, an arc, or a complete brown 

 circle in one or more annual rings of 

 the wood. 



Because two other common diseases 



of the elm produce similar symptoms, 

 positive identification of the Dutch 

 elm disease depends upon laboratory 

 tests that involve identification of the 

 fungus that may grow from the dis- 

 colored wood. Without these tests, 

 the Dutch elm disease cannot be dis- 

 tinguished with certainty from other 

 wilt diseases of elm. A laboratory to 

 which specimens may be sent for 

 identification of Dutch elm disease is 

 maintained by the Bureau of Ento- 

 mology and Plant Quarantine of the 

 Department of Agriculture. 



Ceratostomella ulmi develops in liv- 

 ing trees as a parasite and in dead elm 

 wood as a saprophyte. In living trees, 

 the fungus occurs in water-conducting 

 vessels of the wood. It produces yeast- 

 like spores that are carried through 

 these vessels in the flow of sap. The 

 toxins the fungus produces and the 

 brown, gumlike deposits in the water- 

 conducting vessels cause wilt and the 

 death of the tree or its affected 

 branches. After its host dies, the fungus, 

 still growing on the wood as a sapro- 

 phyte, produces spores under the loos- 

 ened bark and in insect galleries 

 formed between the bark and wood. 



OCCASIONALLY THE FUNGUS spreads 

 through linkage of diseased and 

 healthy trees by natural root grafts, 

 which frequently occur in dense elm 

 stands and crowded street plantings. 

 Normally, though, the fungus is borne 

 from diseased trees to healthy trees by 

 two kinds of bark beetles, the native 

 elm bark beetle, Hylurgopinus rufipes 



